1. Field of the Invention
The present invention relates to an oxygen sensor element which can be used in an air-fuel ratio control for an automobile engine, and further relates to a method of producing such an oxygen sensor element.
2. Description of the Prior Art
In the exhaust system of an automobile engine, an oxygen sensor is provided for measuring the oxygen concentration in the exhaust gas so as to perform an air-fuel ratio control based on measured values of the oxygen concentration.
The oxygen sensor includes an oxygen sensor element for detecting the oxygen concentration. The oxygen sensor element includes a solid electrolyte and electrodes provided on the solid electrolyte. The electrodes include an internal electrode exposed to a reference gas and an external electrode exposed to a gas to be measured.
For providing the foregoing electrodes at electrode forming portions of the solid electrolyte, the following method has been used:
First, noble metal nuclei are adhered to the electrode forming portions of the solid electrolyte to form nucleus forming portions. Then, metal plating is applied to the nucleus forming portions to form plating films. Thereafter, the plating films are burned to form the foregoing electrodes on the solid electrolyte.
The foregoing nucleus forming portions are achieved by spraying particles of noble metal, such as platinum (Pt), onto the electrode forming portions of the solid electrolyte.
As shown in FIGS. 24A and 24B, a large number of fine holes or cavities are formed on the surface of the solid electrolyte. Accordingly, when the noble metal particles are applied onto the surface of the solid electrolyte at the electrode forming portions thereof, the noble metal particles enter the cavities of the solid electrolyte to form the nucleus forming portions. Thus, upon plating the nucleus forming portions, a plating liquid reacts with the noble metal particles within the cavities so that the plating films are organically tangled with particles of the solid electrolyte to achieve a strong adhesion force therebetween based on an anchor effect. Then, by burning the plating films, the electrodes are achieved which are hard to peel off from the surface of the solid electrolyte.
However, there is the following problem in the foregoing method of forming the nucleus forming portions:
Specifically, there may be formed a complicated pattern, as the foregoing electrode, on the surface of the solid electrolyte as shown, for example, in FIG. 2A and FIG. 9. Accordingly, in the foregoing forming method employing spraying of the noble metal particles, it is necessary to partially mask the surface of the solid electrolyte and thus it is difficult to produce the electrode of a complicated shape.
Japanese First (unexamined) Patent Publication No. 4-95766 discloses another forming method, wherein a solution containing a noble metal compound is applied to the electrode forming portions of the solid electrolyte to form coating films and then, by heating the coating films at a high temperature, those components (for example, a binder) other than the noble metal in the solution are volatilized or decomposed so that only the noble metal nuclei are deposited to form nucleus forming portions.
In the latter forming method, the nucleus forming portion can be easily provided at the electrode forming portion of a desired shape using, for example, screen printing, stamp printing, pad printing, roll transfer, dip method, spray method or dispenser method.
However, the latter forming method has the following problem:
Specifically, since the heating of the coating films is carried out at a high temperature, i.e. about 700.degree. C. or higher, flocculation of the noble metal advances so that the mean particle diameter of the noble metal nuclei becomes 0.1 .mu.m to 0.8 .mu.m. Hence, as shown in FIG. 24A, the noble metal nuclei 92 can not enter the fine cavities 21 formed on the surface of the solid electrolyte 2, but stay at entrances of the fine cavities 21.
In this case, as shown in FIG. 24B, the plating film 119 can not advance into the fine cavities 21 so that the adhesion force based on the anchor effect can not be achieved between the plating film 119 and the solid electrolyte 2.
Further, as shown in FIG. 24A, in the latter forming method, the noble metal nuclei 92 are localized on the surface of the solid electrolyte 2. This means that distances between the adjacent noble metal nuclei 92 become large. As appreciated, the adhesion force between the plating film 119 and the solid electrolyte 2 can not be achieved at portions where no noble metal nuclei 92 exist, and thus, in the latter forming method, those portions where the adhesion force can not be achieved exist largely on the surface of the solid electrolyte 2.
Consequently, in the latter forming method, such an oxygen sensor element tends to be produced, wherein the peeling-off of the electrode is liable to occur and the surface resistance at an interface between the electrode and the solid electrolyte 2 is excessively large to disable outputs required for detection of the oxygen concentration.